Einstein's Gravitational Waves May Set Speed Limit for Pulsar Spin

Washington, D.C. -- Gravitational radiation, ripples in the fabric of space predicted
by Albert Einstein, may serve as a cosmic traffic enforcer, protecting reckless pulsars from
spinning too fast and blowing apart, according to a report published in the July 3 issue of
Nature.

Pulsars, the fastest spinning stars in the Universe, are the
core remains of exploded stars, containing the mass of our
Sun compressed into a sphere about 10 miles across. Some
pulsars gain speed by pulling in gas from a neighboring star,
reaching spin rates of nearly one revolution per millisecond,
or almost 20 percent light speed. These "millisecond" pulsars
would fly apart if they gained much more speed.

Using NASA's Rossi X-ray Timing
Explorer,scientists have found a limit to how fast a pulsar spins and speculate that
the cause is gravitational radiation: The faster a pulsar spins, the more gravitational radiation
it might release, as its exquisite spherical shape becomes slightly deformed. This may restrain
the pulsar's rotation and save it from obliteration.

"Nature has set a speed limit for pulsar spins," said Prof.
Deepto Chakrabarty of the Massachusetts Institute of
Technology (MIT) in Cambridge, lead author on the journal
article. "Just like cars speeding on a highway, the fastest-
spinning pulsars could technically go twice as fast, but
something stops them before they break apart. It may be
gravitational radiation that prevents pulsars from destroying
themselves."

Gravitational waves, analogous to waves upon an ocean, are
ripples in four-dimensional spacetime. These exotic waves,
predicted by Einstein's theory of relativity, are produced by
massive objects in motion and have not yet been directly
detected.

Created in a star explosion, a pulsar is born spinning,
perhaps 30 times per second, and slows down over millions of
years. Yet if the dense pulsar, with its strong gravitational
potential, is in a binary system, it can pull in material
from its companion star. This influx can spin up the pulsar
to the millisecond range, rotating hundreds of times per
second.

In some pulsars, the accumulating material on the surface
occasionally is consumed in a massive thermonuclear
explosion, emitting a burst of X-ray light lasting only a few
seconds. In this fury lies a brief opportunity to measure the
spin of otherwise faint pulsars. Scientists report in Nature
that a type of flickering found in these X-ray bursts, called
"burst oscillations," serves as a direct measure of the
pulsars' spin rate. Studying the burst oscillations from 11
pulsars, they found none spinning faster than 619 times per
second.

The Rossi Explorer is capable of detecting pulsars spinning
as fast as 4,000 times per second. Pulsar breakup is
predicted to occur at 1,000 to 3,000 revolutions per second.
Yet scientists have found none that fast. From statistical
analysis of 11 pulsars, they concluded that the maximum speed
seen in nature must be below 760 revolutions per second.

This observation supports the theory of a feedback mechanism
involving gravitational radiation limiting pulsar speeds,
proposed by Prof. Lars Bildsten of the University of
California, Santa Barbara. As the pulsar picks up speed
through accretion, any slight distortion in the star's dense,
half-mile-thick crust of crystalline metal will allow the
pulsar to radiate gravitational waves. (Envision a spinning,
oblong rugby ball in water, which would cause more ripples
than a spinning, spherical basketball.) An equilibrium
rotation rate is eventually reached where the angular
momentum shed by emitting gravitational radiation matches the
angular momentum being added to the pulsar by its companion
star.

Bildsten said that accreting millisecond pulsars could
eventually be studied in greater detail in an entirely new
way, through the direct detection of their gravitational
radiation. LIGO, the Laser Interferometer Gravitational-Wave
Observatory now in operation in Hanford, Wash. and in
Livingston, La., will eventually be tunable to the frequency
at which millisecond pulsars are expected to emit
gravitational waves.

"The waves are subtle, altering spacetime and the distance
between objects as far apart as the Earth and the Moon by
much less than the width of an atom," said Prof. Barry
Barish, LIGO director from the California Institute of
Technology, Pasadena. "As such, gravitational radiation has
not been directly detected yet. We hope to change that soon."

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